In a reflector antenna[?], the relative reduction in the effective aperture of the antenna caused by the masking effect of other antenna parts, such as a feed horn or a secondary reflector, which parts obstruct the radiation path.
... out with 17.4% under the age of 18, 3.5% from 18 to 24, 22.3% from 25 to 44, 28.7% from 45 to 64, and 28.1% who are 65 years of age or older. The median age is 50 ...